There are many industrial processes in which materials are treated at elevated temperatures to alter their surface characteristics, and thereby their overall properties. It is of course advantageous to continuously monitor the surface of the material, and the ambient gas conditions, in situ during manufacture, to derive such information as will enable optimal process control. Particular advantage in this regard would inure from the provision of a single instrument that is capable of making non-contact measurements to obtain all relevant data simultaneously and on-line during production, and of means for employing the totality of the information obtained so as to achieve improved accuracy and to realize the self-evident benefits that are attendant thereto.
An especially notable application for such process-monitoring technology lies in the semiconductor industry, where the need exists for reliably determining, and thence controlling, the physical and electronic properties of thin film structures in the course of production. The required information is known to be contained in the optical film constants, from which knowledge rather crude empirical correlations have been developed that relate film properties to, for example, the index of refraction (n.sub..infin.) in the visible range. That parameter is typically measured however at a wavelength that is remote from the absorption features that correlate directly its film composition (e.g., by reflectance or ellipometry), and is therefore of only limited value.
The determination of temperature, and certain related properties, by making non-contact measurements is well known in the art. Thus, in U.S. Pat. No. 4,172,383, Iuchi discloses a device for measuring the temperature and emissivity of a heated, specular reflecting material. U.S. Pat. No. 4,417,822, to Stein et. al., provides a laser radiometer. In U.S. Pat. No. 4,456,382, Iuchi et. al. disclose a device for measuring the temperature and emissivity of a heated material knowing the temperatures of both the ambient surroundings and also the furnace. In accordance with Tank U.S. Pat. No. 4,974,182, the temperature and emissivity of an object are obtained by making radiance measurements from the object at multiple frequencies and at different temperatures, while it is either heated or cooled, so as to eliminate the influences of the temperature and absorption characteristics of the surrounding ambient. U.S. Pat. No. 4,919,542, to Nulman et. al., teaches a technique by which the emissivity and temperature of an object having zero transmission are determined by measurements of radiance and reflectivity, the ambient radiation for a selected range of processing temperatures being accounted for by making calibration measurements in which the true sample temperature is determined by, for example, contact thermometry.
The determination of layer thickness by optical means is also described in the art. In U.S. Pat. No. 4,555,767, to Case et. al., the infrared reflectivity of an object is measured to determine the thickness of an epilayer of known composition; this is done by comparing the measured reflectance to values of theoretical reflectance determined for different thickness of an epilayer and the substrate.
The prior art also provides optical techniques for determining the composition of a material. In a publication by Buffeteau and Desbat, entitled "Thin-Film Optical Constants Determined from Infrared Reflectance and Transmittance Measurements" (Applied Spectroscopy, Vol. 43, No. 6, 1989, pages 1027 through 1032), the authors describe a general method, based upon reflectance and transmittance measurements, for the determination of the optical constants, n(v) and k(v), of thin films deposited upon any substrate, transparent or not. The corresponding computer program involves three main parts: (1) a matrix formalism to compute reflection and transmission coefficients of multilayered systems; (2) an iterative Newton-Raphson method to estimate the optical constants by comparison of the calculated and experimental values; and (3) a fast Kramers-Kronig transform to improve the accuracy of calculating the refractive index. It is disclosed that the first part of the program can be used independently to simulate reflection and transmission spectra of any multilayered system using various experimental conditions.
In U.S. Pat. No. 4,791,296, to Carpio, the measurement of infrared transmission is employed in a dual beam apparatus to determine the phosphorous concentration in PSG and BPSG films. This is accomplished by comparison of a transmission measurement, made through a substrate upon which a film is deposited, to a transmission measurement of the substrate alone; the measured transmittance difference is compared to calibration curves. The American Society for Testing Materials has issued standards to measure carbon (Standard No. F-121-76) and oxygen (Standard No. F-123-74) (1) using IR absorption spectroscopy. In U.S. Pat. No. 4,590,574, to Edmonds, the oxygen in wafers having one rough surface is measured by employing the shape of an IR transmission to determine roughness, and to thereby correct the measurement of the oxygen or carbon absorption peaks for the affect of the rough surface. The dielectric function of a surface, obtained by ellipsometry over the range of frequencies 1.5 eV to 6 eV, is employed by Aspnes et. al. in U.S. Pat. No. 4,332,833 to determine the microstructure of the material.
Determination of the end point of a plasma etch process, applied to a surface, has been accomplished by noting changes in the character of the ambient above the surface. Thus, in U.S. Pat. No. 4,455,402, to Gelernt et. al., a spectrophotomatic observation of the plasma is employed using a single detector; in U.S. Pat. No. 4,695,700, to Provence et. al., two detectors are utilized; and in U.S. Pat. No. 4,493,745, to Chen et. al., a simulation is employed to predict the etch end point from a measured part of the optical emission spectroscopy.
A gas temperature measurement system, operating upon absorbed and emitted radiation, is described in Tandler et. al., U.S. Pat. No. 2,844,032. In U.S. Pat. No. 2,878,388, Bergson discloses a system for analyzing gases by measuring the absorption of radiant energy.
It is known that laser interferometry can be employed to monitor etch depth in a plasma reactor, using a method that relies upon the time dependence of the diffraction from a patterned substrate to determine the depth of etching as a function of time. Also, in situ ellipsometry has previously been performed on dielectric layers to measure thickness, and light scattering has been utilized to measure film thickness in a MOCVD reactor. Infrared interference techniques have been successfully applied to epitaxial GaAs layers in making film thickness measurements.
Mass spectrometry is known to be a highly sensitive technique for measuring ion concentrations and energy distributions at a surface. Optical emission spectroscopy uses visible radiation emitted by the plasma as a diagnostic, and laser-induced fluorescence can be used to yield relative concentration, and some temperature, information.
Dispersive IR instruments have been employed to measure gas phase absorbances outside of a plasma processing zone, and IR lasers have been proposed for making in situ measurements. In limited instances, in situ Fourier Transform Infrared (FTIR) spectroscopy has been applied to plasma processing; e.g., to record high resolution spectra of a SiH.sub.4 plasma in both emission and absorption, and to study the plasma etching of SiO.sub.2 by CF.sub.4. Spectrometers have also been employed to investigate N.sub.2 O plasmas; in situ studies have been made on films of amorphous hydrogenated silicon, using a dispersive IR spectrometer and polarizing the IR to remove gas absorptions; and several authors have reported making in situ IR measurements on films of a-Si:N:H, a-Si:F(H), and SiO.sub.2.
Despite the prior art exemplified by the foregoing, the need exists for a fast and accurate method, and a unitary apparatus for performing the same, by and with which a multiplicity of characteristics of a substrate surface undergoing modification can be determined, in situ and simultaneously, for effective control of the processing conditions.